Finding something new to say about John F Kennedy isn’t easy. He was only in the White House for two years, 10 months and two days, yet more ink has been spilt about him than any other President. Is there any point in yet another biography?

Most books about the 35th President of the United States fall into one of two categories: they either perpetuate the Camelot myth or debunk it. Thurston Clarke tries to straddle both these camps, providing plenty of material about Kennedy’s womanising exploits (exhibit A in the case for the prosecution), but asserting that he would have gone on to be a great President if he hadn’t been assassinated on that fateful day in Dallas.

Clarke has his work cut out, not least because his focus is on the last hundred days of Kennedy’s Presidency. That means he can only refer in passing to the Cuban missile crisis, which was surely JFK’s finest hour.

The only episode Clarke dwells on that falls outside this three-month period is the death of Patrick Kennedy, the President’s son who was born six weeks premature. Clarke claims that this tragedy strengthened JFK’s marriage to Jackie and led him to see the error of his philandering ways.

Is that true? One of the problems with JFK’s Last Hundred Days is that it’s impossible to disprove Clarke’s case. His argument is that [itals] had he lived [itals] Kennedy would have become a better President and a better man. Well, yes, maybe, but how can we ever know? It reads a bit like one of those “What if?” books that amateur historians occasionally write in which they imagine how the world might have turned out if some pivotal event had never happened.

For instance, Clarke claims that many of the triumphs that are usually chalked up to Lyndon B Johnson, Kennedy’s successor, properly belong to JFK. Thus, according to him, the 1964 Civil Rights Act would have found itself on to the statute books even if Kennedy hadn’t been killed on 22nd November 1963 and he finds plenty of evidence to support this assertion in Kennedy’s final days.

“What is clear is that just as ambition and realpolitik had characterized his congressional career and early White House years, morality and emotion tempered his ambitious during his last hundred days,” write Clarke.

In fact, Kennedy’s Civil Rights Bill had little chance of being passed when he was still President because it was being held up by the Southern Democrats in the Senate who regarded JFK as a Harvard Liberal. It took a good ol’ boy like Lyndon Johnson to get them on side and he was only able to do that by engaging in some old-fashioned horse-trading and ruthlessly exploiting Kennedy’s death to ratchet up the political pressure. He presented the Civil Rights Bill as JFK’s “legacy” and dared them to stand in its way. But the irony is, it wouldn’t have been part of his legacy if LBJ hadn’t been there to pilot it through the Senate.

Given Kennedy’s shortcomings, it’s hard to believe he would have achieved all that Johnson went on to achieve if he’d remained in office, let along the other miracles Clarke thinks he would have pulled off. These include ending the Cold War, dismantling the world’s nuclear arsenal, fixing the American economy and continuing to inspire the young by “marrying poetry to power”. Why not throw in finding a cure for cancer while you’re at it?

Clarke’s problem is that Kennedy’s record in office was distinctly patchy. He was, at best, a mediocre Senator and all the evidence points to his election victory in 1960 having been bought for him by his powerful and corrupt father. He wasn’t just a bit of a lad when it came to the ladies, he made spectacularly bad choices. One of his mistresses was sleeping with Sam Giacama, the mafia kingpin, and another was a KGB agent.

Yes, he handled himself well during the Cuban missile crisis, but his policy of détente towards the Soviet Union, later taken up by Nixon, helped to prop up a murderous, bankrupt regime for another 25 years. As President, he certainly had an ambitious legislative programme, but because of his preference for making grandiloquent speeches over the dirty business of making deals, most of it languished in Congressional committees. It took a politician with a sure grasp of realpolitik – precisely the quality Clarke derides – to see it become law.

The faith Clarke displays in the golden boy of American politics is touching, but the transformation he detects in JFK’s last 100 days is almost entirely in his imagination. Johnson was twice the President Kennedy was or was every likely to be and this book does nothing to prove otherwise.